System for monitoring and controlling substances in fluid bodies

ABSTRACT

The addition of substances, such as chlorine, to fluid bodies is controlled and the concentration of such substances is monitored by a system including a temperature compensated probe for measuring the concentration, and electronic means responsive to outputs of the probe for adding the substance, as needed, to the desired level.

. United States Patent [1 1 1111 3,809,922 Emmons et al. 5] May 7, 1974 SYSTEM FOR MONITORING AND 3,652,910 3/1972 Urbain 307/116 CONTROLLING SUBSTANCES IN FLUID 3,378,479 4/1968 Colvin et a]. 204/248 BODIES 3,481,857 12/1969 Gray 204/272 [76] Inventors: Donald R. Emmons; William C.

Bevefly Jr. both of 11222 Primary Examiner-Robert K. Schaefer Astronaut Blvdqoflando, Fla Assistant Examiner-M. Ginsburg 3 0 Attorney, Agent, or Firm-Duckworth, Hobby & Allen 22 Filed: Nov. 10, 1972 [21] Appl. No.1 305,309 [57] ABSTRACT I The addition of substances, such as chlorine, to fluid [52] US. Cl. 307/118, 137/3 bodies is controlled and the concentration of Such Illt. Cl. Substances is monitored a System including a [58] Field of 307/116 210/96 perature compensated probe for measuring the con- 200/ 6104 6105 centration, and electronic means responsive to outputs of the probe for adding the substance, as needed, to [56] References d the desired level.

UNITED STATES PATENTS 2,999,797 9/1961 Campbell... 210/62 X 12 Claims, 3 Drawing Figures 10 B1) (30 n J 31 SYSTEM FOR MONITORING AND CONTROLLING SUBSTANCES IN FLUID BODIES BACKGROUND OF THE INVENTION takes place in an economical manner. For example, the

chlorine concentration in swimming pools is generally kept at between 0.5 3.0 parts per million (ppm).

In many cases swimming pool and other water supply operators are required to monitor the chlorine level several times daily, to insure that the proper level is kept. Often, this monitoring operation simply comprises a sample on which a chlorine test is administered by the operator. The chlorine is then added mechanically or manually to adjust the chlorine concentration. However, this type of operation is expensive and time consuming.

Automatic station-keeping systems for monitoring and controlling the addition of chlorine have been devised. See for example U. S. Pat. Nos. 3,481,857 to Gray and 3,378,479 to Colbin et al. Other techniques are also disclosed by Baker and Griffin, in the Journal of the AWWA, April, 1958, at page 489; and by Grune and Chueh, at page 479 of Volume 30, Sewage and Industrial Wastes, 1958.

SUMMARY OF THE INVENTION The present invention comprises a system and method for monitoring and controlling substances in a fluid body. The system comprises a circuit element adapted to conduct current therethrough, and means electrically coupled to the circuit element and adapted for switching the circuit element in response to an input signal thereto. The system includes means for predetermining a given input signal value for which the switching means switches the circuit element, and means for adding the substance in response to current flow through the circuit element.

THE DRAWINGS FIG. 1 is a block diagram of a system in accordance with the present invention;

FIG. 2 is a schematic circuit diagram of a portion of the system of FIG. 1; and

FIG. 3 is a cross section of a probe employed in the system of FIG. 1 and in conjunction with the circuit of FIG. '2.

DETAILED DESCRIPTION A system for monitoring and controlling the chlorine concentration in a swimming pool is shown in FIG. 1 and described with reference thereto. While chlorine andthe chlorine concentration in swimming pools is described in this embodiment, it will be understood that the system may be used to monitor and control the level of a variety of substances.

The system, referred to generally as 10', includes a swimming pool 12 adapted to contain water 14 therein. A chlorine monitoring chamber 16 and a separate chlorine adding chamber 18 communicate in series with each other and with the swimming pool-l2 via piping 20, such that water flow is initially out of the swimming pool, into the monitoring chamber 16, through the adding chamber 18 and thence is returned to the swimming pool. Water flow is sustained by pumping means (not shown).

A chlorine level probe 22, hereinafter described in greater detail with respect to FIGS. 2 and 3, is disposed in the flow path of water flowing through the monitoring chamber 16. The probe 22 is adapted to provide an electrical signal input to a control circuit 24, which input is proportional to the level of the chlorine in the water in the monitoring chamber 16.

The control circuit 24 is adapted to receive the input signal from the probe 22 and, if the input signal indicates that chlorine is below a desired level, the control circuit initiates an energizing signal via circuit lines A and B to a motor associated with a chlorine storage facility 26, such that chlorine is added in the chlorine adding chamber 18 in order to raise the level of the chlorine. Further, the control circuit 24 includes an adjustable control means which allows the desired chlorine level controlled by the system to be predetermined and thereafter controlled at that level of the system. The control means may include a coarse adjustment 28 and a fine adjustment 29, for example. The circuit 24 also includes indicating means, such as a meter 30, for visually indicating the concentration of chlorine in the water, as measured by the probe 22. In the present embodiment, the circuit includes a'meter control 31 andindicating bulbs B1 and B2.

An embodiment of the control circuit 24 of the present invention is shown in FIG. 2. The circuit 24 includes a variety of active and passive circuit. elements, including capacitors (C), resistors (R'),'transistors (Q), integrated circuits (IC), triac (TR), diodes (D), ammeter (M), inductor (L), transformer (T), reed switch (RS), switch (S), bulbs (B), fuse (F), and a standard AC cable (J), all of which are designated by the appropriate capital letter or letters and corresponding numeral, in a manner well known in the electronics art. A parts list of the circuit configuration is attached hereto as APPENDIX 1; it will be understood, how ever, by those skilled in the art that various modifications and changes with respect to specific elements and sub-combinations of elements can be made in the circuit 24 without departing from the scope of the present invention. I

In the circuit 24, 1 15 volt AC line voltage is steppeddown to 34 volts in transformer T1, with 17 volts between each side' of the secondary winding and the grounded center tap. Diodes Dl-4, transistors Q1 and 2, resistors Rl-4, capacitors C1-4 and Zener'diodes This positiveinput is identified as terminal 3 in accordance with standard operational amplifier notation, in which the negative and positive input terminals of the operational amplifier are identified by the numerals 2 and 3, respectively; the output terminal by 6, and the negative and positive terminals by 4 and 7, respectively. Operational amplifier [C1 is a voltage follower having the output terminal 6 shorted to the negative input terminal 2.

The' circuit 24 further comprises an NPN transistor Q3 having the emitter terminal thereof coupled to the [2 volt bus line 32 through resistor R6. The cathode of Zener diode D' is coupled to the [2 bus line 32, with the anode thereof tied to the base of transistor Q3. The base of transistor Q3 is tied to ground through resistor R5. The collector of transistor Q3 is coupled to the second terminal 38 of the probe 22. i

The first terminal 36 of probe 22 is coupled to the cathode of the diode D6, which is physically located 1 within the probe 22, as hereinafter described in greater detail with reference to FIG. 3. The anode of diode D6 is coupled to the negative input terminal 2 of the second operational amplifier [C2 through resistor R13. The transistor Q6 is diode-connected (base connected to collector) to the base of transistor Q7, with the collector of transistor Q7 coupled to the node connecting the anode of the diode D6 and resistor R13. The base of transistor Q6 is coupled to ground through resistor R12. The emitters of transistor Q6 and Q7 are coupled to the [2 volt bus line 32 with the emitter of transistor Q7 being coupled through resistor R23. The two transistors Q6 and Q7 and the associated resistors R12 and R13 are a constant current source which provides a relatively stable current into the diode D6 in order to bias that diode. As noted in the Appendix, the same transistor type is selected for transistors Q6 and Q7 in order to achieve a high degree of temperature compensation.

Transistor O4 is likewise diode-coupled to transistor Q5, and with the associated resistors R7 and R8 providing a constant current source in a manner like that described above with respect'to transistor Q6 and Q7. The collector of transistor Q5 is coupled to the cathode of Zener diode D7, the anode of which is coupled to ground. Potentiometers R10 and R11 are connected in series with each other to ground and in parallel with the Zener diode D7. The variable wiper of potentiometer R10 is coupled to the positive input terminal 3 of the second operational amplifier [C2, and the wiper of the potentiometer R11 is grounded. As thus configured, the circuit of transistors Q4 and Q5 provide a constant current to the positive terminal 3 of the second operational amplifier [C2, and thus establish a reference voltage thereto which biases the entire circuit at the operating point determined by that reference voltage. This reference voltage level can then be varied and predetermined by adjustment of the potentiometers R10 and R11, as hereinafter described. The second operational amplifier [C2 comprises a gain amplifier, the gain of which is determined by the feedback circuit comprising resistor R14 and capacitor C5 connected in parallel between the output terminal 6 and negative input terminal 2 thereof. The third operational amplifier [C3 is similarly connected as a gain amplifier, in which the gain is determined by the feedback loop of resistor R17 and capacitor C6. The output t terminal 6 of the second operational amplifier [C2 is 4 coupled to the negative input terminal 2 of the third operational amplifier [C3 through resistor R15, and the output terminal 6 of the first operational amplifier [C 1 is coupled to the positive input terminal 3 of the thir operational amplifier through resistor R16.

The output terminal 6 of the third operational amplifi er [C3 is directly coupled to the negative input terminal 2 of the fourth operational amplifier [C4. While the fourth operational amplifier [C4 is described as an operational amplifier, that device serves as a threshold'detector, and any integrated circuit or similar circuit arrangement capable of open loop voltage gain operation and with an extremely high minimum voltage gain will suffice.

Diodes D8 and D9 are reverse-poled relative to each other in parellel between ground and the positive input terminal 3 of the fourth operational amplifier [C4. Resistor R21 is coupled between the output terminal 6 and the positive input terminal 3 of the fourth operational amplifier [C4. In the negative input circuit of the fourth operational amplifier [C4, resistors R18 and R19 are coupled in series with each other to the negative input terminal 2 thereof, with the ammeter M1 coupled between resistors R18 and R19 to ground. In this embodiment, ammeter M1 corresponds to the meter 30 of FIG. 1. Resistor R19 is coupled to the wiper terminal of potentiometer R20, which is coupled between the RS1, which is in turn coupled to resistor R22 between t the gate and a first vone of the main terminals of the triac TRl. The other main terminal of the triac TRl is coupled to one side of the line voltage A, and a bulb B1 is coupled between the two main terminals of the triac. The first main terminal is connected to one prong of a three pronged AC cable J1, the other prong of which is coupled to the other side of line voltage (B). A bulb B2 is coupled between the two current conducting prongs of the line cord. v

The probe 22 will now be described with reference to FIG. 3. The probe 22 comprises a hollow core ferrule nut 50 formed of an insulating material, and adapted to be'mounted on the wall of the chlorine monitoring chamber 16 (note FIG. 1). A hollow core male connecter 52 extends into the ferrule nut 50, as shown in FIG. 3. A first tubular electrode 54 is mounted on the male connecter 52 and extends out of the core of the connecter. A second tubular electrode 56 is mounted coaxially in the first metal electrode 54 and extends out of the first electrode in the same direction that the first electrode extendsout of the connecter 52. An insulating tube 58, such as teflon, is interposed be tween the first and second electrodes 54 and 56.

The first and second electrodes 54 and 56 comprise two materials having dissimilar contents. Preferably, the electrodes 54 and 56 comprise two dissimilar stainless steel alloys. By way of example, it has been found that electrode 54 may comprise stainless steel alloy No. 304 and electrode 56 may comprise stainless steel alloy No. 416.

A second hollow insulating tube 60 is disposed inside the second electrode 56. Diode D6 is disposed inside the extended end of the second electrode 56, with the cathode of that diode electrically coupled to the second electrode, as with a conductive epoxy 62, for example. The anode of diode D6 extends through the second insulating tube 60. A three-pin connecter 64 is fitted inside the ferrule nut 50, and includes three conductive pins 66, 68 and 70 extending therethrough. A first one of the pins 66 corresponds to the second terminal 38 of FIG. 2 and is electrically coupled to the first, or outermost electrode 54 via a wire 72. The inner pin 68 is electrically coupled to the anode of diode D6 via a wire 74 extending through the second hollow insulating tube 60. The third pin 70 is electrically coupled to the second, or innermost electrode 56 by a wire 76. The three pins 66, 68 and 70 are interconnected with the circuit 24 of FIG. 2 by a cable (not shown).

The manner in which the probe 22 and the circuit 24 monitors and controls the amount of chlorine added in the swimming pool 12 will now be described.

With the switch S1 closed and the circuit 24 energized, the constant current circuit including transistors Q6 and Q7 provides a stable supply current to diode D6 and a bias current between the electrodes 54 and 56 of FIG. 3. As the water flows past the electrodes 54 and 56, the dissimilar materials of the electrodes and the chlorine act in combination in a known manner as an electrolytic cell, the voltage level of which is dependent upon the oxidation level of chlorine in the water.

In accordance with the present invention, since diode D6 is biased at a constant current level and is in the path of water flow, then the voltage across the diode changes with respect to variations in thetemperature of the water. Further Zener diode D5, transistor Q3 and the associated circuitry provide a small signal current which bucks the current flow between electrodes 56 and 54 (to ground). In the past, one difficulty with electrolytic probes has been the tendency for oxide coatings to form on the electrodes and alter and degrade monitoring characteristics of the probe. By providing a bucking current between the electrodes 56 and 54 in the probe 22, thesedetrimental coatings are largely prevented.

Noting FIG. 2, transistors Q4 and Q5 and Zener diode D7 provide a stable voltage reference of 6.0 volts to ground. Zener diode D7 is preferably a very stable compensated device with respect to temperature variations (see Appendix 1). This 6.0 volt reference voltage is divided through the series resistance of potentiometers R10 and R11. The wipers of potentiometers R10 and R11 are manually adjusted by the operator to achieve a coarse (R10) and then a fine (R11) adjustment to the desired level of chlorine in the water supply (in parts per million). Potentiometers R10 and R11 thus correspond to the coarse and fine adjustments 28 and 29 of the control circuit 24 as shown in FIG. 1. The reference voltage predetermined by the settings of potentiometers R10 and R11 then establishes a predeter- 'mined operating point for the circuit 24.-

As noted above, the voltage produced between the electrodes 54 and 56 of the probe 22 reflect the amount of chlorine in the water,'which voltage biases the input terminal 3 of the operational amplifier IC3 (via the voltage following operational amplifier 1C1). Simultaneously, the voltage potential between the electrodes'54 and 56 appears between the negative and positive input terminals 2 and 3 of the third operational amplifier 1C3. Since the output of the second operational amplifier IC2 is referenced to the operating point established by the bridge defined by potentiometers R10 and R11, then the output from the operational amplifier 1C3 is analagous to the difference between the voltage across those potentiometers with respect to the voltage across the electrodes 54 and 56 of the probe 22. This analagous output from operational amplifier 1C3 is positive or negative, depending upon whether the voltage across the electrodes 54 and 56 is above or below the level of the reference voltage determined by the potentiometers R10 and R11.

Referring now to the fourth operational amplifier 1C4, the two reverse-poled parallel-connected diodes 1 D8 and D9 create a limiting reference voltage into the positive input terminal 3 of that operational amplifier. In this embodiment, that reference voltage has a magnitude of about 0.6 volts, the polarity of which is determined by the output of the operational amplifier 1C4. The resistor R21 provides a positive feedback into the positive input terminal 3.

' In operation, the analagous output from the operational amplifier 1C3 reflects minute changes in the chlorine level, as detected by the probe 22. When the magnitude of this output slowly goes beyond the magnitude of the reference voltage into the positive input terminal 3 (0.6 volts in this example), the operational amplifier 1C4 becomes saturated and its output changes polarity; that is, if the input to terminal 2 is negative and drops below the reference voltage level, then the output jumps positive and is fed back into the positive input terminal 3 through resistor R21, causing the reference voltage to terminal 3 to change polarities. In this manner, the output of operational amplifier [C4 is latched at either a positive or negative voltage level for minute changes in the analagous output of the operational amplifier 1C3, which reflects the changes in chlorine content. For example, it has been found that this latching function is achieved in the circuit 24 when the output of operational amplifier IC3 changes as little as a millivolt with respect to the reference voltage into terminal 3 of the operational amplifier 1C4.

If the output of the operational amplifier 1C4 reflects that the chlorine level is below the desired level, then that output flows into the inductor L1, which closes the reed switch RS1. With the reed switch RS1 closed, current flows into the gate of triac TRl causing current to flow between the main terminals of that triac. This in turn closes the circuit between line voltage and the circuit lines A and B into the motor 25 in the chlorine storage facility 26. When the motor 25 in' the chlorine storage facility 26 is energized, chlorine, either as a dry particulate, a liquid solution or in gaseous form, is added to the water in the chlorine adding chamber 18 (note FIG. 1 Conversely, when the probe 22 indicates that the amount of chlorine in the water is at the desired level, the output of the operational amplifier 1C4 opens the reed switch RS1 which shuts down the triac TRl, thus preventing the further addition of chlorine to the water. Bulb B1 indicates when current is not flowing through the triac TRl, and bulb B2 indicates when line voltage is present. It will be appreciated by those skilled in the art that the circuit 24 may also be used with a motor speed control circuit to control the speed at which the chlorine is added.

Again noting FIG. 2, the ammeter M1 is coupled through the resistor R18 between the negative input terminal 2 of the operational amplifier 1C4 and ground. In this manner theoutput of the operational amplifier 1C3 is visually indicated on the meter M1, again, which corresponds to meter'30 in FIG. 1. Resistor R19 and potentiometer R20 provide a positive bias to the meter, so that the meter continuously reads positive, regardless of whether the output of operational amplifier [C3 is positive or negativerPotentiometer R20 corresponds to the meter control 31 of FIG. 1.

The present invention provides an automatic stationkeeping system which monitors and accurately controls the level of substances added to fluid bodies. The probe of the present invention accurately monitors the level of the substances, with the output of the probe being compensated with respect to temperature changes of the fluid. Further, the probe of the present invention can be usedfor long periods of time since the deposition of harmful coatings on the electrodes of the probe 'is largely avoided. The present invention also comprises a circuit which will control the additions of the substance in accordance with minute changes in the level of the substance, and will latch into a switched condition when such minute changes are detected.

Another advantage of the system of the present invention inures by virtue of indications on the ammeter Ml. Most large swimming'pools employ a number of adjustable water jets spaced around the perimeter of the pool. When any one or more of these jets are inoperative or improperly adjusted, dead" pockets within the pool are created which do not include the required amount of chlorine. As the water is drawn through the present system, these dea'd. pockets will appear as distinct meter fluctuations, indicating to the operator that the water jets are not functioning properly. Further, air leaks on the suction side of the intake of the present system will be indicated by rapid oscillations on the meter, thus'providing a continual evaluation as to the integrity of the pumping means.

APPENDIX I Rl.2: 470 ohm RSA: 2200 ohm R5: i000 ohm R6: 39 k ohm R7: 390 ohm R8: 39 ohm R9: 10 k ohm R10: 0-2.5 k ohm potentiometer R] 1: 0-250 ohnt'potcntiomcter R12: 1000 ohm R13: 3) k ohm R14: 100 k ohm R15: 6.8 k ohm R16: 6.8 k ohm R17: 500 k ohm R18: 6.37 k ohm, 1% R19: H) k ohm R20: 0.10 k ohm potentiometer R21: H) k ohm R22: 270 ohm R23: 56 ohm Cl,2: I50 microfarad. 40 volt (3,4: 100 microfarad, 35 volt (35.6: 0.0! microfarad Ql: TIP 29 (Texas lnst. Co.) PNP or equivalent Q2: TlP 30 (Texas lnst. Co.) NPN or equivalent .Q3,4.5,6.7: 2N3906 NPN or equivalent lCl,2,3,4: 741 operational amplifiers or equivalent 4 TRl: 2N5755 triac or equivalent D1.2.3.4: lN4002 D5: lN825 Zener D6: lN400l D7: lN825 Zener D8.9,l0: 1N400l D1]: lN 759A'Zener D12: IN 759A Zener Ml: O-l m.a.Ammeter L1, R81: 900 ohm coil. pull in reed switch at 9 volts. drop out at 1 volt. or

8 equivalent 1 15 volt primary, l7 volt-to-center tap secondary, secondary currcn rated at 300 milliamps, or equivalent. AGC-3 ofa positive and negative polarity prior to latching;

and means for latching said output by swinging said reference voltage to the other polarity when said input signal value exceeds the magnitude of said reference voltage. 2. Apparatus as recited in claim 1 wherein said switching'means comprises: 7

an input terminal for receiving said input signal;- another input terminah'and means for providing said reference voltage to said switching means via said another input terminal.

3. Apparatus as recited in claim 2 wherein said reference voltage means comprises two diodes reverse poled and coupled in parallel with each other between said another terminal and ground.

4. Apparatus as recited in claim 3 wherein said latching means comprises said two diodes and resistive means between said another and output terminals.

5. Apparatus as recited in claim 1 wherein said predetermining means comprises:

means coupled to said switching means for providing said input signal thereto;

means for varying said reference voltage; and

wherein p said input signal to said switching means is analagous to the difference between said reference voltage and a signal input to said input signal providing means.

6. Apparatus as recited in claim 5 wherein said reference voltage providing means comprises:

a temperature stabilized Zener diode; and circuit means for providing a constant current into said Zener diode.

7. Apparatus as recited in claim 6 wherein said vary-- ing means comprises variable resistance means in parallelwith said Zener diode. i i

8. A system for monitoring and controlling the level of a substance in a fluid'body, comprising:

means for monitoring the level of said substance and for providing corresponding electrical signals;

means for processing said signals and controlling means'for adding further amounts of said substance to said fluid body, said processing means comprising:

a circuit element adapted to conduct current therethrough;

v9 means having an output electrically coupled to said circuit element for switching current through said circuit element in response to said signals of a predetermined value; means for providing a reference voltage to said switching means, said reference voltage being one of a positive and negative polarity prior latching; means for latching said output by swinging said reference voltage to the other polarity when said predetermined signal value exceeds the magnitude of said reference voltage; and means associated with said processing means for predetermining a given signal value from said monitoring necessary to activate said adding means. 9. A system as recited in claim 8 wherein said predetermining means comprises:

circuit means for receiving said signals from said monitoring means; means for varying said reference voltage; and

wherein said circuit means is adapted to provide an input to said switching means which is analagous to the difference between said reference voltage and said signals from said monitoring means.

10. A system as recited in claim 9 further comprising means associated with said switching and circuit means for visually indicating said analagous input.

11. A systemas recited in claim 10 wherein said visual indicating means comprises:

an ammeter coupled to the output of said circuit means; and

means for biasing said ammeter continuously positive.

12. A system as recited in claim 8 wherein said means for swinging the polarity of said reference voltage comprises a feedback impedence between said output of said switching means and said reference voltage provid- 

1. Apparatus comprising: a circuit element for conducting current therethrough; means having an output electrically coupled to said circuit element for switching current through said circuit element in response to an input signal to said switching means; means for predetermining a given input signal value for which said switching means switches said circuit element; means for providing a reference voltage to said switching means, said reference voltage being one of a positive and negative polarity prior to latching; and means for latching said output by swinging said reference voltage to the other polarity when said input signal value exceeds the magnitude of said reference voltage.
 2. Apparatus as recited in claim 1 wherein said switching means comprises: an input terminal for receiving said input signal; another input terminal; and means for providing said reference voltage to said switching means via said another input terminal.
 3. Apparatus as recited in claim 2 wherein said reference voltage means comprises two diodes reverse poled and coupled in parallel with each other between said another terminal and ground.
 4. Apparatus as recited in claim 3 wherein said latching means comprises said two diodes and resistive means between said another and output terminals.
 5. Apparatus as recited in claim 1 wherein said predetermining means comprises: means coupled to said switching means for providing said input signal thereto; means for varying said reference voltage; and wherein said input signal to said switching means is analagous to the difference between said reference voltage and a signal input to said input signal providing means.
 6. Apparatus as recited in claim 5 wherein said reference voltage providing means comprises: a temperature stabilized Zener diode; and circuit means for providing a constant current into said Zener diode.
 7. Apparatus as recited in claim 6 wherein said varying means comprises variable resistance means in parallel with said Zener diode.
 8. A system for monitoring and controlling the level of a substance in a fluid body, comprising: means for monitoring the level of said substance and for providing corresponding electrical signals; means for processing said signals and controlling means for adding further amounts of Said substance to said fluid body, said processing means comprising: a circuit element adapted to conduct current therethrough; means having an output electrically coupled to said circuit element for switching current through said circuit element in response to said signals of a predetermined value; means for providing a reference voltage to said switching means, said reference voltage being one of a positive and negative polarity prior latching; means for latching said output by swinging said reference voltage to the other polarity when said predetermined signal value exceeds the magnitude of said reference voltage; and means associated with said processing means for predetermining a given signal value from said monitoring necessary to activate said adding means.
 9. A system as recited in claim 8 wherein said predetermining means comprises: circuit means for receiving said signals from said monitoring means; means for varying said reference voltage; and wherein said circuit means is adapted to provide an input to said switching means which is analagous to the difference between said reference voltage and said signals from said monitoring means.
 10. A system as recited in claim 9 further comprising means associated with said switching and circuit means for visually indicating said analagous input.
 11. A system as recited in claim 10 wherein said visual indicating means comprises: an ammeter coupled to the output of said circuit means; and means for biasing said ammeter continuously positive.
 12. A system as recited in claim 8 wherein said means for swinging the polarity of said reference voltage comprises a feedback impedence between said output of said switching means and said reference voltage providing means. 